130 



THE HUMAN EMBRYO. 



the very broad medullary plate of thickened ectoderm. Toward the middle 

 of its length the medullary plate is somewhat narrower than elsewhere. 

 Along its median line runs the deep, narrow, dorsal groove which at its 

 caudal end widens out and disappears. Just behind it is the opening of the 

 relatively large neurenteric canal, behind which again follows a remnant of 

 the primitive groove. A transverse section a little in front of the middle of 

 the embryo is shown in figure 26. The ectoderm, ek, is very much thickened 

 to constitute the medullary plate; the narrow central longitudinal furrow, 



Ent. 



FIG. 65. HUMAN EMBRYO OF 1.54 MM. MEDIAN SEC- 

 TION FROM A WAX MODEL RECONSTRUCTED FROM 

 SECTIONS. 



All, Allantois. Am, Amnion. b. s, Body-stalk. C/io, Chor- 

 ion. EC, Ectoderm. Ent, Entoderm. mts, Mesoderm. 

 Vi, Chorionic villus. Yk, Cavity of yolk-sac. (After 

 Count Spee.) 



b--' 



FIG. 66. HUMAN EMBRYO OF 1.54 MM. 

 Transverse section passing the neurenteric canal 

 and yolk-sac, am, Amnion. ek, Ectoderm. 

 ct, Amniotic mesoderm. g, Meeting-point of 

 somatopleure and splanchnopleure. df, 

 Mesoderm of yolk-sac, b, b, b, Blood-ves- 

 sels, en, Entoderm. n, Neurenteric canal. 

 d, Cavity of yolk-sac, e, Medullary plate. 

 {After Count Spec.} 



/, mentioned above is very noticeable. Outside of the embryo the ectoderm is 

 reflected on to the amnion, Ct, over the back of the embryo. The entoderm is 

 a thin layer of cells in the center of which the notochordal band can be distin- 

 guished, ch. In sections near the neurenteric canal the notochord is better 

 marked, being there much thicker than the remaining entoderm. The meso- 

 derm, me, is a distinct layer, although, as other sections show, it is fused in the 

 median line of the primitive streak behind the neurenteric canal with both 



